There is a great need to determine the condition of certain electrical equipment such as generators and motors. High voltage generators such as are used by electric power utilities are subject to faults and deterioration which can be relatively easily corrected if detected sufficiently early but which can lead to catastrophic failure if left uncorrected. A variety of techniques have been employed to monitor the condition of such equipment in order to evaluate whether or when maintenance should be performed. Such techniques generally measure, directly or indirectly, the quality of the electrical insulation of the equipment. A high voltage generator, such as a 13.2 kV generator used by an electrical utility, includes insulated conductors which are disposed in slots in laminated steel structures. During generator operation, the insulation is subjected to a very large cyclic compressive load. This is a result of the mechanical forces generated by the interaction between the electrical current in the conductor and the magnetic field. Such stresses, together with time and environmental conditions, eventually result in deterioration of the insulation sufficient to permit corona generation. For purposes of the present invention, the term "corona" means a localized electrical discharge in a gaseous region adjacent an electrical circuit which occurs due to transient ionization of the gas when the voltage stress in the gas exceeds a critical value, for instance an electric field of about 3000 V/m in air. The corona itself typically causes further insulation degradation which, if unabated, will result over time in an arc, spark or flame in the equipment which causes an equipment failure. Thus, for purposes of the present invention, a corona or corona ionization may be thought of as a first step in an evolutionary process which, in time, may result in an arc, spark or flame in the equipment being monitored. Although the term "corona" may be used to refer to internal discharges, such usages of "corona" must be distinguished from corona which occurs externally. External corona refers to characteristics of the corona phenomenon which occur on the surface of the insulation. External corona is sometimes referred to as partial discharge. Corona that is not visible because it is internal to a material or device can also be referred to as partial discharge. In this specification, "corona" refers to external corona, i.e, exposed ionization. While certain measures can be taken to suppress external corona ionization, such as applying semiconductive paint over the insulation, they generally merely extend the time until corona occurs.
In rotating machinery, such as high-voltage motors or generators, conductors are insulated with layers of insulating tape which is mica based and impregnated with a polymer that has a high dielectric strength which is inherent to the monomer components of the polymer or to the modifiers which may be added to increase the dielectric strength upon polymerization of the monomer compound. Conductors that have been treated with an insulating system are driven into the slots in laminated steel structures. These stator-bar conductors must carry a cyclic compressive load. This is a result of the transmission of mechanical forces resulting from the interaction of the current and magnetic field to the iron of the generator or the opposite affect. Semiconductive paints are used to suppress corona between the insulation system and the slots. In most cases, the paint is vaporized over prolongation of the ionization. Corona is undesirable and will degrade the insulation due to ion bombardment over time.
Currently, insulation can be divided into six classes. However, these six classes have yet to become the industry standard. The industry is currently using four class divisions. Class A insulation is comprised of organic materials such as cotton, paper and silk. This type of insulation is not used in new generators. Class B insulation is a group consisting of synthetic materials such as mica, glass fiber and epoxy. Some organic materials are also included in this class. Class B insulation is the most widely used insulation today. Class F insulation is made up of similar materials that are in Class B insulation. These materials have different characteristics than those in Class B. Generally, Class F insulation can withstand an additional 25 degrees centigrade over Class B insulation. Class H insulation is also comprised of similar materials as in Class B insulation. Class H insulation has additional additives that suppress combustion. The proposed six class division separates Class H insulation into two other classifications that are loosely based on further temperature divisions. Insulation systems are usually mica or mica based products which have been impregnated with a plastic in a compression process. Insulating tapes used in an insulation system that covers the outer surface of conductors contain inorganic materials, even so, corona ionization will eventually occur on such materials.
Corona is a serious problem, and can lead to rapid and catastrophic failure of a generator. Accordingly, generator condition monitoring has included corona detection and monitoring of corona-related conditions. For the most part, prior corona detection techniques have required the generator to be taken out of service, which is a substantial disadvantage. One technique is to visually inspect the generator windings to detect the effects of corona on the insulation; corona may leave a white residue. Visual inspection may require substantial disassembly of the generator to access locations where corona may occur, and the visual evidence of corona may be overlooked. Another corona detection technique which requires the generator to be out of service is electrical measurement of the insulation electrical characteristics from which inferences regarding the insulation quality and susceptibility to corona may be drawn. D.C. potentials may be applied to the windings for measurements of charging and static resistive currents; A.C. potentials may be applied to the windings to make insulation power factor measurements; or overvoltages may be applied to determine if the insulation can survive them. A further drawback of these electrical testing methods is that they do not provide information regarding the physical location at which an insulation problem may exist; thus they do not provide information to direct repair and maintenance activities to the appropriate locations. Other techniques may be used to detect corona based on the effects it produces while it is occurring; these include detecting radio or ultrasonic noise emitted by the corona. These monitoring techniques also require the electrical load to be removed from the generator, and it is still difficult to locate a corona-generating region using them. Coronas generate ozone, and the tell-tale odor of ozone has long been used as an indicator of corona. It is difficult to quantify the degree of insulation deterioration and to localize a deteriorated area by detecting corona-generated ozone.
Corona also generates light, and corona detectors which acquire and detect light have been used in research studies regarding corona physics. However, to date optical corona detectors have not been available which can detect corona in an operating electrical machine such as a generator and determine the location where the corona exists.
Utility generators can be taken off line for inspection, testing, maintenance, and repair in the spring and fall, when demand is relatively low, but it is inconvenient and expensive to do so in the summer and winter peak demand seasons. Accordingly, the generators are typically kept on line during the peak seasons, and any minor insulation defects which are overlooked or are incipient and undetectable in the spring and fall can grow to serious or catastrophic problems during the following peak season. Such problems are exacerbated by the fact that there is presently negligible construction of new power plants in the United States; old plants and their generators are therefore being kept on line indefinitely, and the aging equipment is increasingly susceptible to insulation deterioration. When a catastrophic generator failure occurs, it can require eighteen months off line and tens of millions of dollars to repair.
Accordingly, there is a great need for a system for continuously monitoring the condition of a generator while it is in normal operation, which can detect and evaluate the severity and location of corona and related conditions, so that problems can be identified early, monitored during their evolution, emergency action can be taken if necessary, and otherwise repair and maintenance can be scheduled when appropriate, performed efficiently and effectively, and orderly utility operations maintained.
It is therefore an overall object of the invention to provide a system for detecting corona in electrical machinery while the machinery is in normal operation.
It is a further object of the invention to provide a system which can detect arcing, flame ignition combustion, and smoldering conditions as well as corona.
It is another object of the invention to provide a system for detecting corona ionization emissions from an electrical power generator, which system can be installed on the electrical power generator without modifying or altering the design or structure of the generator.
It is another object of the invention to provide a system which can determine the location(s) within the machinery at which corona or other insulation defect-related conditions are occurring.
It is another object of the invention to provide such a system which can determine the identity of materials which are involved in corona or other insulation defect-related conditions.
It is another object of the invention to provide such a system which can determine the intensity or severity of corona or other insulation defect-related conditions.
It is another object of the invention to enhance the optical visibility of corona ionization emissions that occur within electrical power generating equipment by treating the insulation used in such equipment with materials which emit a distinctive optical signature during corona ionization emissions.
It is another object of the invention to provide such a system which displays information relating to the intensity and location of corona or other insulation defect-related conditions.
It is another object of the invention to use a single optical sensor which is coupled to a plurality of fiber optical devices with different fields of view for monitoring and determining the location(s) within the machinery where corona ionization emissions are occurring. It is a still further object of the present invention to provide means for selectively suppressing optical signals from one or more of such fiber optical signals from reaching the single optical sensor. It is yet a further object of the present invention to provide means for optically isolating different optical signals from the plurality of fiber optical devices on discrete non-overlapping areas of the single optical sensor.
It is another object of the invention to provide such a system which stores information relating to the condition of the monitored electrical machinery.
It is another object of the invention to provide such a system which is rugged and can reliably operate in the hostile environment of many electrical machines.
The foregoing and other objects of the invention will be understood with reference to the following specification and claims and the drawings.